Crayfish escape behavior and central synapses. II. Physiological mechanisms underlying behavioral habituation.

responsible for exciting the lateral giant neuron and initiating single tail flips in response to phasic mechanical stimuli to the tail of crayfish. It was shown that the excitation of some tactile interneurons by tactile afferents antifacilitates extensively at low repetition rates. This phenomenon must be presumed to contribute to the habituation of the response. It is not clear, however, that this is the only phenomenon responsible for generating lability in the behavior. Receptor fatigue, variable properties of the excitable membranes of the lateral giant or the tactile interneurons, or labile properties of the circuit efferent to the giant are additional possibilities. One other point of lability has in fact been found. The strength of transmission at the neuromuscular junction between the motor giant neuron and the phasic flexor muscles is very sensitive to stimuli recurring only once per minute; this junction rapidly ceases to transmit activity after only a few stimuli (6). The motor giant is sometimes excited by the lateral giant (20, 39), and so it appears that this is a source of declining response strength in the efferent limb of the neural circuit mediating escape. However, this loss of transmission is adequately compensated by the continued activation of several nongiant flexor motoneurons, whose neuromuscular junctions facilitate (33, 39). Furthermore, electrical stimulation of the lateral giant axon at frequencies up to 5 Hz can elicit up to 50 apparently normal tail flips (unpublished observations; see also ref